Alpine belly is an old term. It just means carrying a reserve of fat that can be utilized like a camel's hump. It doesn't mean someone is obese, just that they have some extra, and on men that often presents itself in the belly region. On women I suppose it would "alpine hips" or "alpine ass".

It's not always obvious though. Many skinny people carry a kind of invisible fat in their abdomens, "visceral" fat. It's been in the news recently because of the risks associated with it (cardiovascular, metabolic, cancer).

Without actually measuring your body fat it would be hard to know. But either way it sounds like you have a lower than average base metabolism. That's a big advantage on long mountain trips, but potentially an issue in city life where food is overly abundant and tends to be highly caloric.

I've always had the opposite problem. I'm adapted to a world where there's a food source every 50 feet.